OK - get some thick card, cut into strips around 30 - 40 mm wide x the length of the screen. Make a few test posi's with a lot of small text all over it - around 10 - 14pt in coreldraw and fine lines. Set it up to cover the whole screen with text and get it ready to expose then place the card strips ontop of the glass to block all the light apart from a 30mm strip. Switch your lights on and leave for around 3 mins then every minute take a strip of card off and repeat until you get to around 20 minutes - mark the screen with the minutes exposed.
The 1st part of the screen will have had the longest exposure at 20 mins then every strip will have 1 minute less. Wash the screen out and you'll see the best exposure time. The screen will probably be un-usable and need washing off but its an acurate way to get your timings right.
Simples
Cheers John